With respect to exhaust gas purification means, various attempts have heretofore been made as follows:
{circle around (1)} exhaust gas purification means for using a catalytic body carrying a metal having a catalytic function in partition walls of a honeycomb structural body to decompose components such as hydrocarbon, carbon monoxide, and nitrogen oxides in exhaust gas by oxidation-reduction reaction;
{circle around (2)} exhaust gas purification means for trapping and removing particulate materials in the exhaust gas by the partition walls in a structure in which the partition walls of the honeycomb structural body are constituted of porous materials, predetermined through channels are plugged at one end of through channels formed by the partition walls, and the remaining through channels are plugged at the other end; and
{circle around (3)} exhaust gas purification means for adsorbing/removing hydrocarbon components in exhaust gas by adsorption layers which are formed of zeolite, activated carbon, and the like and which are disposed on the partition walls of the honeycomb structural body.
Moreover, in recent years, as exhaust gas regulation is strengthened, there has been a demand for a higher purification performance, and an exhaust gas purification system by a combination of the above-described different exhaust gas purification means has been developed as one of the attempts which meet the demand.
For example, in JP-A-7-232084, a honeycomb structural body has been disclosed in which a columnar honeycomb structural body is bonded to a hollow cylindrical honeycomb structural body via a low-thermal expansion ceramic bond material and the honeycomb structural bodies have different partition wall thicknesses and cell densities.
However, in this honeycomb structural body, it has been necessary to separately manufacture the respective honeycomb structural bodies so that shapes of both the bodies are precisely matched with each other. This complicates manufacturing steps and increases manufacturing costs, and looseness, disengagement, and the like of a bonded portion caused by mismatch between both the shapes have easily occurred.
Moreover, in the honeycomb structural body, stress is easily concentrated on the bonded portion, and there has been a problem that the looseness, disengagement, and the like of the bonded portion are easily caused by mechanical and thermal shocks during transport between the steps, or coating of the catalyst or an absorbent material, canning or actual using, and the like.
On the other hand, in WO01/04466, a honeycomb structural body has been disclosed in which a catalytic body is directly integrated with a filter.
However, in this honeycomb structural body, a technique of simultaneously and integrally manufacturing bodies of materials having different characteristics has not heretofore been developed with respect to a ceramic honeycomb structural body. For this and another reasons, although performances required by the catalytic body and the filter are largely different from each other, the whole honeycomb structural body is constituted of the materials having the same characteristics, and original performances demanded for the catalytic body and filter cannot sufficiently be fulfilled.
Moreover, in the same publication, an exhaust gas purification apparatus is disclosed including: the honeycomb structural body in which the catalytic body is formed integrally with the filter; and a metal case in which the honeycomb structural body is held and which includes an exhaust gas introductory tube abutting on a portion of the catalytic body of the honeycomb structural body.
However, in this exhaust gas purification apparatus, a honeycomb section which is a portion allowed to function as the catalytic body is allowed to abut on the introductory tube for introducing the exhaust gas so as to secure a path of the exhaust gas, but the end of the honeycomb structural body allowed to abut on the introductory tube is flat. Therefore, when the end of the honeycomb structural body is continuously loaded with large thermal shock and vibration, the exhaust gas leaks out of a desired path via a gap between the introductory tube and the end, and a sufficiently purification performance cannot sometimes be obtained.